Silicon photonics devices based on Silicon-on-Insulation (SOI) have been widely developed. A silicon photonics device typically has a waveguide which is coupled to an optical fiber so that an optical signal from the optical fiber can be coupled into the waveguide. The waveguide of the silicon photonics device generally has very small dimensions, e.g. only several hundred nanometers (i.e. a small mode size). On the other hand, an optical fiber e.g. a single mode fiber generally has a large diameter (i.e. a large mode size). Thus, there is usually a mismatch between the mode size of the waveguide and the mode size of the single mode fiber when the waveguide is directly coupled with the single mode fiber. Therefore, the coupling loss may be very high if the nano-scale silicon waveguide directly couples with the single mode fiber. In addition, the alignment tolerance may be very small. Therefore, it may be difficult to package silicon photonics devices with single mode fibers.
In view of the above-mentioned problems, a low loss optical converter for the silicon photonic devices was developed to couple the waveguide and the optical fiber. In this low loss optical converter, a silicon nano-scale tip is covered with a low refractive index silicon dioxide layer. The optical converter may have a coupling loss of about 2.0-3.0 dB when it is coupled with a tapered fiber. However, the optical converter may have a higher coupling loss and a small alignment tolerance when coupled with a single mode fiber.
A low loss polymer converter was also developed. The polymer converter may have a polymer core covering a silicon nano-scale tip. The polymer converter may have a low loss of about 0.8 dB when coupled with an optical fiber. However, the process for manufacturing the polymer converter may not be compatible with complementary metal-oxide-semiconductor (CMOS) technology. The same polymer converter was then fabricated using silicon oxynitride (SiON) as the cladding waveguide. However, the coupling loss for this polymer converter may be high, for example about 2.5 dB.
A vertical grating coupler can be used for coupling the waveguide and the optical fiber. Generally, the vertical grating coupler has a good alignment tolerance. However, the vertical grating coupler may be dependent on wavelength, and the process for manufacturing the vertical grating coupler may be difficult to control.